/Engine/Source/Runtime/Engine/Classes/GameFramework/GameModeBase.h
also PlayerController::GetSeamlessTravelActorList() (the function that's called on clients)
virtual void GetSeamlessTravelActorList ( bool bToTransition, [TArray](API\Runtime\Core\Containers\TArray)< [AActor](API\Runtime\Engine\GameFramework\AActor) * > & ActorList )